This lambda is a custom resource provider implemented in golang.
AWS Cloudformation invokes the lambda during the creation of the stack and the lambda will return the ARN of a load balancer created in a Elastic Beanstalk environment.
The example-template.yaml file shows how to setup this functionality.
In that file there are 2 resources, one is the AWS::Serverless::Function to define the lambda, and the other is the Custom::DescribeElbEnvironment, this is what invokes the lambda during cloudformation execution, returning the load balancer's ARN. The lambda is invoked by the !GetAtt GetLbArn.LoadBalancerArn construct in the template file.